Mammalian mitochondria contain S55 ribosomes of unusual composition and physical-chemical properties. The objective of this research project is the elucidation of the structure, composition, function and biosynthesis of mammalian mitochondrial ribosomes to allow a comprehensive comparison of S55 ribosomes with extramitochondrial ribosomes. Mitochondrial and microsomal ribosomes prepared on a large scale from bovine liver will be used, with E. coli S70 ribosomes, in comparative studies of antibiotic sensitivity and ribosome function in cell-free systems. The ribosomal proteins of S55 and extramitochondrial ribosomes will be enumerated and characterized by polyacrylamide gel electrophoresis in two dimensions. These analyses will include studies of MW, stoichiometry, amino acid composition, topographical distribution and functional assignments. Experiments will also be performed to ascertain which, if any of the mitochondrial r-proteins are made in mitochondria, and also to determine functional homologies between r-proteins of S55 and S70 E. coli ribosomes. BIBLIOGRAPHIC REFERENCES: T.W. O'Brien and D.E. Matthews, "Proteins of the Bovine S55 Mitochondrial Ribosome," J. Cell Biol. 63:210a (1974). D.E. Matthews, N.D. Denslow and T.W. O'Brien, "Characterization of Bovine Mitochondrial Ribosomes Prepared by Ion-Exchange Chromatography", Fed. Proc. 34:502 (1975).